Generally hearing aids are equipped with a vent allowing sound pressure equalisation between ambient and the residual space between the tympanic membrane and ear-mould, ICI, or ITE hearing aid. The vent may also prevent occlusion experienced by the user of the hearing aid, which occlusion is caused by enclosed sound waves conducted via the skull and head tissue to the residual space. The vent ensures that the enclosed pressure changes may be equalised with ambient pressure.
However, the introduction of the vent has a downfall. The acoustic properties of the vent may cause a large leakage of low frequency energy undermining the low frequency gain target of the hearing aid, and, in fact, establish a positive feedback loop between the loud speaker and the microphone. While the leakage of low frequency energy is often compensated by increasing the low frequency gain care should be taken such as to avoid an unstable positive feedback situation.
Hearing aid fitting software uses data tables and/or physical models of the vent for estimating the acoustic properties of the vent during a fitting session with dispenser. However, modelling of the acoustical properties of the vent is very difficult due to an insufficient parametric description of the vent as it has been produced in the actual ITE aid or BTE ear-mould and also the real-ear impedance of the ear and the residual space. Therefore resulting effect of the vent and of interaction between the actual vent and the actual ear is not modelled accurately in the model of an average ear and vent.
In this context the term “dispenser” is to be construed as a person fitting a hearing aid to a user, such as a medical doctor, an audiologist, or any adequately trained person.
Manufacturers of hearing aids store acoustic properties of the vent in the hearing aid. In case the acoustic properties stored in the hearing aid do not correspond to the physical vent this leads to large errors in the prescription and simulation of the hearing aid and hence leads to a poor fitting of the hearing aid to the user. Even if the acoustic properties are measured and stored correctly there is still a large variety in the precise physical shape of the vent, and therefore a variety of possible vent responses.
In addition, the acoustic properties of the vent may be measured by dispenser by means of a Real-Ear-Measurement (REM). This measurement is performed by inserting a microphone in the residual space to measure the sound pressure level at the tympanic member. The dispenser may correlate the results of the REM with the acoustic properties stored in the hearing aid, but the dispenser is not able to change the acoustic properties stored in the hearing aid. Furthermore, the probe causes a change of the residual space and the insertion of the probe as such causes a leakage, which leads to incorrect results.
In view of the problems of introducing a vent in the ear-mould, CIC or ITE it is of utmost importance to design the dimensions of the vent carefully.
Various prior art documents describe feedback cancellation techniques overcoming the above described disadvantages of positive feedback such as caused by the vent. For example, American patent application number US 2001 0002930, which is hereby incorporated in the present specification by reference, discloses a hearing aid comprising feedback cancellation means including means for estimating a physical feedback signal of the hearing aid, and means for modeling a signal processing feedback signal to compensate for the estimated physical feedback signal. The hearing aid further comprises subtracting means, connected to the output of the microphone of the hearing aid and to the output of the feedback cancellation means, for subtracting the signal processing feedback signal from the audio signal to form a compensated audio signal. Hence the feedback cancellation means compensate for feedback introduced by, for example, the vent size of an ear-mould for a BTE hearing aid, a CIC, or a ITE hearing aid. However, the American patent application does not perform an identification of possible causes for the generated positive feedback.